1. Field of the Invention
The present invention relates to disk drives. More particularly, the present invention relates to a servo pattern which is printed on a disk of a disk drive. Even more specifically, the present invention relates to a hybrid servo pattern, including gray code and chevrons, both of which are printed on a disk of a disk drive, and to a method of positioning a head relative to the disk by reading the hybrid servo pattern.
2. Relevant Background
Disk drives that magnetically record, store, and retrieve information on disk-shaped storage media, i.e., disks, are widely used in the computer industry. A magnetic head includes a write transducer and a read transducer. The write transducer is used to record information on the disk, and the read transducer is used to retrieve information from the disk. A magnetic head assembly includes the head and an air bearing slider that suspends the magnetic head assembly relative to the rotating disk by “flying” on a small cushion of air proximate to the disk surface when a spindle supporting the disk is rapidly rotated.
The magnetic head assembly is mounted on the end of an actuator arm that is moved by a voice coil motor to position the head radially relative to the disk surface. The head reads data from and writes data to generally concentric tracks located on the disk surface. Each concentric track has a unique radius, and reading and writing information to or from a specific track requires the head to be located above the track. By moving the actuator arm, the head is moved radially relative to the disk surface across various tracks.
Disk drives typically use embedded servo sectors of magnetically recorded information which are stored on the disk surface. The servo sectors are read by the head to inform the disk drive of track location. Each track typically includes both servo sectors and data sectors, and the servo sectors are interspersed between the data sectors. Each data sector contains a header followed by a data section. The header may include synchronization information to synchronize various timers in the disk drive to the speed of disk rotation, while the data section is used for recording user data.
Each servo sector (or servo pattern therein) typically includes gray code and several servo bursts. The gray code is a binary code that indexes the radial position of the track such as through a track number and, in some instances, might also include a circumferential index such as a sector number. The servo bursts provide positioning information for the actuator arm and may be a centering pattern that allows positioning the head over the center of the track. By comparing portions of the servo burst signals, the disk drive can iteratively adjust the head location by operating the voice coil motor until a zeroed position error signal is returned from the servo bursts indicating the head is properly centered with respect to the track.
Servo patterns are usually written on the disk using the head. Accordingly, the servo patterns are usually written after the disk drive has become partially operational. The servo patterns, and particularly the track spacing and centering information, need to be precisely located on the disk. However, at the time the servo patterns are written, there are no reference locations on the disk surface which can be perceived by the disk drive.
Typically, a specialized device known as a servowriter (or servo track writer) is used in conjunction with the head when writing servo patterns. Traditional servowriters require two holes in the disk drive. One hole is for receiving a push-pin, which is used in combination with an encoder to guide the actuator arm so that the head is positioned at desired (and precise) radial locations across the disk surface. The other hole is for receiving a clock head, which is used to both write, and repeatedly read, timing information so that servo patterns are written at precise circumferential locations and are aligned in the circumferential direction on all tracks. Since both the radial position and the circumferential position of the head relative to the disk surface are known, it is possible to write servo patterns at precise locations onto the disk surface. Once written, servo patterns serve as positional references on the disk surface for the disk drive's entire life.
Because of the holes required for the servowriter, the servowriter operations must be done in a clean room in order to prevent contamination of the disk drive. However, clean rooms, especially those large enough to accommodate tens of servowriters, are expensive to build and maintain. Furthermore, it is difficult to maintain and operate a servowriter in a clean room. In addition, largely because of the locational precision needed, servowriters are fairly expensive, and servowriting is a time-consuming process that adds to disk drive production costs. Often, the servowriter is the highest-priced device used in the construction of the disk drive, and servowriting is a manufacturing bottleneck.
Servo patterns are conventionally written by moving the head from one radial location to a next radial location until servo patterns have been written across substantially the entire disk surface. That is, the head is positioned at a first radial location and servo patterns are written at that radial location. Subsequently, the head is moved to a next radial location and servo patterns are written at that radial location. This process repeats until servo patterns have been written across substantially the entire disk surface.
As the number of tracks per inch increases, the number of radial positions where servo patterns are to be written increases. In turn, the amount of time that the servowriter requires to write servo patterns increases, thereby exacerbating the bottleneck associated with servowriting. Hence, the data storage industry has begun to search for alternatives to existing servowriting techniques.
Attempts have been made to produce reference disks for servowriting or even disks that eliminate the need for servowriters. These disks have been fabricated using a variety of magnetic and other printing methods that transfer a servo pattern directly to the disk surface prior to insertion of the disk in the disk drive. Some of these printed patterns are narrow rectangular elements and spaces. The resolution of these printing methods has been much lower than that currently achievable with servowriters. Contemporary servo patterns have transition densities of about one eighth micron; however, the transition densities between the narrow rectangular elements and spaces of printed servo patterns are about four to eight times larger. Further, the ends of the narrow rectangular elements have blunted corners which cause the ends to be nearly semi-circular. Such printed servo patterns have blunted corners due to diffusion, diffraction, and other phenomena occurring in the manufacturing processes, and appear as if they had been written by a fat write head with eroded poles. As a result, the disk drive's servo position accuracy is reduced due to fine defects at the conventional ends (or edges) of the servo pattern in such pre-patterned disks.
Pre-printed patterns have been produced with narrow rectangular elements, without gray code. One such pattern is described in U.S. Pat. No. 6,304,407 entitled “Self-Writing of Servo Patterns Based on Printed Reference Pattern in Rotating Disk Drive”, which is incorporated by reference herein in its entirety. U.S. Pat. No. 6,304,407 describes printed reference features (such as inclined, slender rectangular elements) to convert accurate timing measurements into accurate radial position information. This radial position information, along with circumferential timing information from the printed pattern, corresponds to similar data provided by a conventional servowriter; however, it is available when the disk drive is sealed and removed from the costly clean room. Therefore, the control system and other electronics can be assembled to complete the disk drive, and it can be placed on a test rack in a less costly environment. Subsequently, the control system software can write a new, final servo pattern.
While such patterns have proven useful for self-servowriting of disk drives, such patterns have limitations. For example, such patterns can only identify small changes in the radial position of the head relative to the disk surface. Specifically, because discrete Fourier transform measurements provide sine and cosine values from data in a sampling window, a change of one full cycle (360 degrees) gives the same values of the sine and cosine. (Laser interferometers and optical encoders have the same sine and cosine outputs, and the position is only determined within one cycle of 360 degrees.) Therefore, in order to use such patterns, the head motion must be restricted so that more than two samples are made within every 360 degree cycle. Accordingly, the head velocity must be limited so that the sampling rate is not overrun. Hence, high-speed seek operations cannot be performed using such patterns directly. Instead, such patterns are used to write more traditional final servo patterns, with which high-speed seek operations can be performed.
Another limitation of the printed reference patterns described in U.S. Pat. No. 6,304,407 is that such patterns do not provide absolute position information. Rather, such patterns form part of an incremental system which is capable of providing the radial position of the head relative to the disk surface within a 360 degree cycle. Accordingly, if such patterns are used for self-servowriting and if the self-servowriting is interrupted, the head cannot return to the location where the last self-servowritten pattern was written. Instead, the head must return to a known position location (e.g., the crash stop) and creep-in towards the center of the disk in order to complete the self-servowriting. Thus, duplicative efforts may be required to complete the self-servowriting.
Conventional servo patterns include gray code (with one code or numerical value per track) to provide absolute indication of radial position. At any radius, the gray code can be read to provide an approximate track number. However, the gray code is ambiguous at the track boundaries, so ambiguities exist at the track boundaries. The ambiguities in the gray code are no more than one value of the gray code. Conventional servo patterns resolve such ambiguities by comparing the amplitudes of special frequency burst segments (i.e., servo bursts) which are written in each servo sector and positioned off the center of the track. By comparing amplitudes of two or more of the burst segments in a servo sector, it is possible to combine gray code and burst amplitude information to resolve ambiguities in the gray code and, hence, provide absolute position information for any radial position. However, as tracks continue to be made narrower, the ambiguities (or fuzziness due to edge of burst errors) become larger relative to the track width.
Hence, there remains a need for an improved method for producing a printed servo pattern on a disk surface to enable accurate positioning of the head within a disk drive. Such an improved method would preferably be readily reproducible within a manufacturing setting, such as a storage media production facility, while reducing manufacturing time and cost.